This paper explores the role that lines play in information design and their contribution to the development of human communications. It focuses on lines that represent quantitative data but not lines in graphic design that are aesthetic, symbolic or marked in the physical landscape. A number of publications and websites were consulted. These explore information design theory, semiotics, cartography and thematic mapping and showcase the latest innovations in data visualisation.The main part of the paper examines the historical milestones in the development of the line from its first recorded use as concrete representations of figures, then as abstractions of the physical environment in maps and later for metaphorical communication. The last part of the paper mentions some of the different ways to distinguish multiple lines on the same graph, the different forms that the line can take and the symbolic uses of the line. Maps and figures are provided in an annex to demonstrate examples of the key works discussed.This paper concludes that although the line has existed for a very long time, it continually finds new ways to communicate and represent data. Through the history of the line, a series of creative people improved its capacity to carry information. This has been possible due to the versatile form and ambiguous interpretation that the line has to offer. Each innovation has encouraged scientists and innovators to improve on the new technique and take it a step further. They appreciated the value of being able to express complex information and phenomena in a visual format, where the line is key.